Method of planarization

ABSTRACT

A planarization method that utilizes a chemical-mechanical polishing operation. In the polishing operation, a first slurry for polishing a metallic layer is first employed to remove a greater portion of the metallic layer. Next, a second slurry for polishing a dielectric layer and having properties very similar to the metal-polishing slurry is added and mixed together with the slurry for polishing a metallic layer so that the polishing rate for the dielectric layer is increased. Consequently, metallic residues remaining on the dielectric layer are removed, and a planar dielectric layer is obtained at the same time.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of prior applicationSer. No. 09/182,968, filed Oct. 29, 1998.

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] The present invention relates to a method of planarization. Moreparticularly, the present invention relates to a method of forming ametallic plug within an inter-metal dielectric layer by filling metallicmaterial into a via, and then performing a chemical-mechanical polishing(CMP) operation to remove excess metal above the plug and surroundingareas. Once the metallic layer is polished away, metallic residues abovethe inter-metal dielectric layer are removed by a controlled increase inthe polishing rate of the dielectric layer. Consequently, undesirablemetallic residues are completely removed and a planar surface isobtained at the same time.

[0004] 2. Description of Related Art

[0005] Amongst the methods of planarizing a surface, chemical-mechanicalpolishing (CMP) is one of the most important techniques for the globalplanarization for VLSI or ULSI circuit fabrication. In the process ofmanufacturing integrated circuit devices, material often has to beremoved from the surfaces of intermediate devices one or more times.Furthermore, material layers sometimes have to be planarized beforecarrying out the next operation. As the number of material removingoperations or planarization increases, CMP operations are used moreoften. Chemical-mechanical polishing operates by pressing the frontsurface of a wafer down onto a rotating surface on a polishing table,and supplying slurry at the same time.

[0006] The slurry normally contains a chemically active component suchas an acid or a base, and a mechanically active component such asabrasive particles made from silicon dioxide. Although the physicalmechanism of the polishing action is still not fully understood, thechemical reaction and the mechanical abrasion of its active componentshas satisfactorily polished and planarized various types of surfaces.Nowadays, CMP methods are frequently applied to the planarization ofmetallic and dielectric layers.

[0007] A conventional method of forming a plug includes the steps ofdepositing insulating material over an integrated circuit device, andthen patterning the insulating layer to form a contact hole or a viaopening. Finally, conductive material is deposited to fill the hole orthe via so that a vertical interconnect penetrating the insulating layerand linking electrically with a portion of the device in the substrateis formed.

[0008] Since metallic conductive material such as aluminum is unable tofill the via or hole satisfactorily, a chemical vapor deposition (CVD)method is normally used to fill the via of hole with tungsten (W). Inthe process of depositing tungsten into the via, a layer of tungstenalso forms over the insulating layer. After the via is completelyfilled, excess tungsten above the via needs to be removed beforealuminum lines are deposited above the insulating layer and the via. Anetching back method, for example, a reactive ion etching (RIE) method,can be used to remove the tungsten above the insulating layer.

[0009] However, the etching back operation can lead to over-etching, anda portion of the tungsten within the via may be removed to form arecessed cavity. Therefore, subsequently deposited aluminum layer makespoor contact with the tungsten inside the recessed cavity of the via.Furthermore, when the tungsten is etched back, microscopic particleswill remain attached to the wafer surface. These attached particles canoften lead to undesirable damage of the device. Hence, an alternativemethod of removing excessive tungsten above an insulating layer, thatis, a chemical-mechanical polishing method, is often used.

[0010]FIGS. 1A through 1D are cross-sectional views showing theprogression of manufacturing steps in producing a plug according to aconventional method. Due to insufficient planarization of theinter-metal dielectric layer, recessed cavities are produced in someareas leading to device problems.

[0011] First, as shown in FIG. 1A, patterned conductive lines 112 areformed above a semiconductor substrate 100. The conductive lines 112 aremade of aluminum or aluminum alloy. To simplify the drawing, MOS devicesand the semiconductor substrate 100 underneath the conductive lines 112are not fully drawn. Next, a dielectric layer 114, for example, asilicon oxide layer is deposited over the conductive layers 112 and thesubstrate 100. The dielectric layer 114 is somewhat influenced by theunderlying ridge pattern of the conductive lines 112 on the surface ofthe semiconductor substrate 100, such that the surface of dielectriclayer 114 manifests an undulated surface.

[0012] Next, as shown in FIG. 1B, planarization of the dielectric layer114 is carried out. For example, a chemical-mechanical polishing methodis used to form a planarized dielectric layer 114 a. However, not allareas are planarized and some areas contain recessed cavities 116.

[0013] Next, as shown in FIG. 1C, a photoresist layer (not shown in FIG.1C) is formed over the dielectric layer 114 a. The photoresist layerexposes portions of the dielectric layer 114 a where via openings willform. Thereafter, the dielectric layer 114 a is selectively etched usingan etching technique such as a dry etching method. Ultimately, a portionof each conductive line 12 is exposed, forming via openings 126 and adielectric layer 114 b.

[0014] In the subsequent step, a glue layer 118 conformal to thedielectric layer 114 b is formed and covers the substrate structure.Thereafter, a layer of tungsten 120 is formed over the glue layer 118and completely fills the via openings 126.

[0015] Next, as shown in FIG. 1D, a chemical-mechanical polishingoperation is carried out using slurry for polishing tungsten. Hence, aportion of the tungsten layer 120 above the dielectric layer 114 b isremoved, thereby forming tungsten plugs 120 a within the via openings126. However, due to the presence of recessed cavities 116 on thesurface of the original dielectric layer 114 b, residual tungsten 120 bis embedded, which makes its removal particularly difficult.

[0016] In fact, because the slurry for polishing tungsten has anespecially slow removing rate for dielectric layer 114 b, residualtungsten 120 b entrenched inside the cavities 116 is difficult to beremoved by prolonging the chemical-mechanical polishing operation. Theresidual tungsten 120 b within the cavities 116 can affect the qualityof a finished device, and may lead to low yield and waste of the siliconwafer.

[0017] In light of the foregoing, there is a need to improve the methodof planarization.

SUMMARY OF THE INVENTION

[0018] Accordingly, the present invention provides a planarizationmethod that utilizes a chemical-mechanical polishing operation. Throughincreasing and controlling the removal rate of the inter-metaldielectric layer, recessed cavities above the dielectric layer due to aninefficient planarization technique are avoided. Therefore, deviceproblems caused by metallic residues remaining on the dielectric surfacecan be reduced.

[0019] In another aspect, this invention provides a planarization methodthat utilizes a chemical-mechanical polishing operation. In thepolishing operation, a first slurry especially for polishing a metalliclayer is first employed to remove a greater portion of the metalliclayer. Next, a second slurry for polishing a dielectric layer and havingproperties very similar to the metal-polishing slurry is added and mixedtogether so that polishing rate for the dielectric layer is increased.Consequently, metallic residues remaining on the dielectric layer areremoved and a planar dielectric layer is obtained at the same time.

[0020] To achieve these and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, theinvention provides a method of planarization. The method includes thesteps of providing a semiconductor substrate having a first metalliclayer, and then forming a dielectric layer over the first metallic layerand the substrate. Next, the dielectric layer is patterned to form a viaopening. Thereafter, a second metallic layer is formed over thedielectric layer and fills the via opening. Subsequently, achemical-mechanical polishing operation using a first slurry forremoving metallic layer is performed to remove most of the secondmetallic layer above the dielectric layer. Then, a second slurry forremoving dielectric layer is added and mixed with the first slurry forremoving metallic layer, while the chemical-mechanical polishingoperation is continued. Eventually, the dielectric layer is planarizedand any residual material from the second metallic layer is removed.

[0021] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary, andare intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings,

[0023]FIGS. 1A through 1D are cross-sectional views showing theprogression of manufacturing steps in producing a plug according to aconventional method;

[0024]FIG. 2 is a cross-sectional view showing a plug fabricatedaccording to one preferred embodiment of this invention; and

[0025]FIGS. 3A through 3D are cross-sectional views showing theprogression of manufacturing steps for producing a plug within aninter-metal dielectric layer having no embedded residual metallicmaterial according to a second preferred embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

[0027] This invention provides a method of removing metallic residuesstop an inter-metal dielectric layer and planarizing the dielectriclayer at the same time. The method is actually a continuation of theconventional process depicted in FIGS. 1A through 1D. By continuing thepolishing operation while adding a slurry for removing dielectric layerto the original slurry for removing metallic layer, the dielectric layer114 b (indicated in FIG. 1D) is planarized. At the same time, a portionof the dielectric layer 114 b together with metallic residues 120 b isremoved.

[0028]FIG. 2 is a cross-sectional view showing a plug fabricatedaccording to one preferred embodiment of this invention. As shown inFIG. 2, the dielectric layer 114 b is now turned into a dielectric layer114 c. Since the dielectric layer 114 b is probably an oxide layer, theslurry used in the chemical-mechanical polishing operation is forremoving oxide. Since operations are carried forward from FIG. 1D,detailed explanations of elements in FIG. 2 are omitted because they arelabeled identically.

[0029] Because the slurry for removing tungsten layer is present whenthe slurry for removing dielectric layer is added, physically similarslurries have to be selected. Physical similarity refers to thecharacteristic of having similar pH value. Therefore, when the two typesof slurries are mixed together, unstable conditions due to a differencein pH values will not happen.

[0030] In other words, the polishing agents within the slurries will notcongeal into lumps that may scratch and damage the wafer. Furthermore,the same type of polishing agents must be used in the two types ofslurries, because different polishing agents will have differentpolishing characteristics. If two or more non-similar types of polishingagents are used at the same time, processing instability may result.

[0031] Hence, according to the embodiment of this invention, pH value ofthe slurry for removing tungsten is around 2.1-2.5, and the polishingagent is silicon dioxide particles with a size of around 200 nm. Itshould be noticed that the slurry for removing tungsten furthercomprises oxidant. The oxidant can be ferric nitrate, hydrogen peroxide,potassium periodate or acid solution containing nitrate group, cyanategroup, periodate group, peroxide group or strong oxidant, for example.The oxidant in the slurry reacts with the surface of the tungsten underacidic condition to form tungsten oxide and the silicon dioxideparticles are used to polish away the tungsten oxide. Therefore, the newtungsten surface is exposed and is oxidized by the oxidant in theslurry. Similarly, in order to match the slurry for polishing tungsten,the slurry for polishing a dielectric has a pH value of around 2.1-2.5,and the polishing agent is again silicon dioxide particles with a sizeof around 200 nm. It should be noticed that the slurry for polishing thedielectric comprises organic acid group such as acetate group, citrategroup and oxalate group. The hydroxide group in the slurry bonds withthe silicon atoms on the surface of the dielectric to form siliconhydroxide which is easily removed by the silicon dioxide particles.

[0032]FIGS. 3A through 3D are cross-sectional views showing theprogression of manufacturing steps for producing a plug within aninter-metal dielectric layer having no embedded residual metallicmaterial according to a second preferred embodiment of this invention.

[0033] First, as shown in FIG. 3A, patterned conductive lines 312 areformed above a semiconductor substrate 300. The conductive lines 312 aremade of aluminum or aluminum alloy. To simplify the drawing, MOS devicesand the semiconductor substrate 300 underneath the conductive lines 312are not fully drawn. Next, a dielectric layer 314, for example, asilicon oxide layer, is deposited over the conductive layers 312 and thesubstrate 300. The dielectric layer 314 is somewhat influenced by theunderlying ridge pattern of the conductive lines 312 on the surface ofthe semiconductor substrate 300, such that the surface of dielectriclayer 314 manifests an undulated surface. However, in this embodiment,planarization of the dielectric layer 314 is deferred until after theplug is formed. By this arrangement, one less chemical-mechanicalpolishing operation can be conducted. Therefore, some wafer processingtime is saved.

[0034] Next, as shown in FIG. 3B, a photoresist layer (not shown in theFig.) is formed over the unplanarized dielectric layer 314. Thephotoresist layer exposes the area where a desired via opening is to beformed. Thereafter, the dielectric layer 314 is selectively etched toform a dielectric layer 314 a using, for example, a dry etching method.Hence, via openings 326 that expose a portion of the conductive line 312are formed.

[0035] In the subsequent step, a glue layer 318 conformal to thedielectric layer 314 a is formed over the substrate structure. Then, aconductive layer 320 is formed over the glue layer 318 and completelyfills the via openings 326. Preferably, the conductive layer is atungsten layer.

[0036] Next, as shown in FIG. 3C, a chemical-mechanical polishingoperation is carried out using slurry for removing metal. Preferably,slurry for removing tungsten is used. Consequently, a large portion ofthe conductive layer 320 above the dielectric layer 314 a is removedforming plugs 320 a. It should be noticed that the slurry for removingtungsten further comprises oxidant. The oxidant can be ferric nitrate,hydrogen peroxide, potassium periodate or acid solution containingnitrate group, cyanate group, periodate group, peroxide group or strongoxidant, for example. The oxidant in the slurry reacts with the surfaceof the tungsten under acidic condition to form tungsten oxide and thesilicon dioxide particles are used to polish away the tungsten oxide.Therefore, the new tungsten surface is exposed and is oxidized by theoxidant in the slurry.

[0037] Next, as shown in FIG. 3D, with the slurry for removing metalpresent, slurry for removing dielectric material is added and mixedtogether with the original slurry. Subsequently, the chemical-mechanicalpolishing operation is continued until the dielectric layer 314 a iscompletely planarized and any metallic residues 320 b are removed fromthe dielectric 314 a surface. It should be noticed that the slurry forpolishing the dielectric comprises organic acid group such as acetategroup, citrate group and oxalate group. The hydroxide group in theslurry bonds with the silicon atoms on the surface of the dielectric toform silicon hydroxide which is easily removed by the silicon dioxideparticles.

[0038] Because the slurry for removing tungsten layer is present whenthe slurry for removing dielectric layer is added, physically similarslurries have to be selected. The reason for choosing physically similarslurries has already been explained in the first embodiment. Therefore,in the second embodiment, pH value of the slurry for removing metal isaround 2.1-2.5, and the polishing agent is silicon dioxide particleswith a size of around 200 nm.

[0039] Similarly, in order to match the slurry for polishing a metal,the slurry for polishing a dielectric has a pH value of around 2.1-2.5,and the polishing agent is again silicon dioxide particles with a sizeof around 200 nm.

[0040] Since planarization using a chemical-mechanical polishing methodaccording to this invention is capable of eliminating metallic residuesabove the surface of a dielectric layer as well as planarizing thedielectric layer, device quality can be greatly improved.

[0041] In summary, major aspects of this invention includes:

[0042] 1. A chemical-mechanical polishing technique is used forplanarization. By first polishing away a large portion of a metalliclayer with slurry for polishing a metal and then adding another similarslurry for polishing a dielectric, the dielectric layer is planarizedand any metallic residues above the dielectric layer are removed at thesame time.

[0043] 2. The slurry for removing metallic material and the slurry forremoving dielectric material are very similar in physical properties.They both have a pH value of between 2.1-2.5, and particles of silicondioxide with a size of around 200 nm are used as polishing agents.

[0044] 3. After the deposition of an inter-metal dielectric layer, stepsin forming a plug can be conducted without the need for performing aplanarization operation first. Hence, one less planarization using achemical-mechanical polishing operation is needed, and some waferprocessing time can be saved.

[0045] It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

What is claimed is:
 1. A planarization method that utilizes achemical-mechanical polishing operation, the method comprising the stepsof: providing a semiconductor substrate that has a first metallic layerformed thereon; forming a dielectric layer over the first metalliclayer; patterning the dielectric layer to form a via opening thatexposes a portion of the first metallic layer; forming a second metalliclayer over the dielectric layer that completely fills the via opening;performing a chemical-mechanical polishing operation, using a firstslurry with an oxidant for polishing the second metallic layer, toremove the second metallic layer above the dielectric layer and form aplug inside the opening; and adding a second slurry containing organicacid groups for polishing the dielectric material without first removingthe first slurry, and then continuing the chemical-mechanical polishingoperation until the dielectric layer is planarized and metallic residuesof the second metallic layer are completely removed.
 2. The method ofclaim 1, wherein step of forming the second metallic layer includesdepositing tungsten.
 3. The method of claim 1, wherein a pH value of thefirst slurry and a pH value of the second slurry are about the same. 4.The method of claim 1, wherein the first slurry and the second slurrycontain the same type of polishing agents.
 5. The method of claim 1,wherein the first slurry has a pH value of between 2.1-2.5, and includesa polishing agent containing silicon dioxide particles with a size ofaround 200 nm.
 6. The method of claim 1, wherein the second slurry has apH value of between 2.1-2.5, and includes a polishing agent containingsilicon dioxide particles with a size of around 200 nm.
 7. The method ofclaim 1, further comprising planarizing the dielectric layer afterforming the dielectric layer.
 8. A planarization method that utilizes achemical-mechanical polishing operation, the method comprising the stepsof: providing a semiconductor substrate that has a first metallic layerformed thereon; forming a dielectric layer over the first metalliclayer; patterning the dielectric layer to form a via opening thatexposes a portion of the first metallic layer; forming a second metalliclayer over the dielectric layer that completely fills the via opening;performing a chemical-mechanical polishing operation, using a firstslurry for polishing the second metallic layer, to remove the secondmetallic layer above the dielectric layer and form a plug inside theopening, wherein the first slurry comprises an oxidant; and adding asecond slurry containing an organic acid for polishing the dielectricmaterial without first removing the first slurry for polishing thesecond metallic layer, and then continuing the chemical-mechanicalpolishing operation until the dielectric layer is planarized andmetallic residues of the second metallic layer are completely removed,wherein a pH value of the slurry for polishing the second metallic layerand a pH value of the slurry for polishing the dielectric layer areabout the same.
 9. The method of claim 8, wherein step of forming thesecond metallic layer includes depositing tungsten.
 10. The method ofclaim 8, wherein the first slurry and the second slurry contain the sametype of polishing agents.
 11. The method of claim 8, wherein the firstslurry has a pH value of between 2.1 to 2.5, and includes a polishingagent containing silicon dioxide particles with a size of around 200 nm.12. The method of claim 8, wherein the second slurry has a pH value ofbetween 2.1 to 2.5, and includes a polishing agent containing silicondioxide particles with a size of around 200 nm.